Piston prover

ABSTRACT

A prover includes a measurement cylinder; a hydraulic cylinder coupled with a side of a downstream end of the measurement cylinder; a measuring piston that moves through a predetermined distance from an upstream side toward a downstream side through the measurement cylinder to eject a reference volume of fluid; and a piston rod that is movably accommodated in the hydraulic cylinder, and has the measuring piston and the piston rod separately constructed. The prover includes measurement standby position stop means, and when returning the measuring piston to a predetermined measurement standby position, the piston rod moves the measuring piston from the downstream side to the upstream side and the measurement standby position stop means stops the measuring piston at the predetermined measurement standby position, after which only the piston rod is caused to move from the upstream side to the downstream side and is accommodated in the hydraulic cylinder.

TECHNICAL FIELD

The present invention relates to a piston prover and, more particularly,to a piston prover having a reduced reference volume for provingaccuracy of a flowmeter.

BACKGROUND OF THE INVENTION

A proving device for proving a flowmeter is a device for subjecting anewly manufactured flowmeter or a flowmeter in use to a characteristictest periodically or at any timing in order for usage at a reliableaccuracy regardless of a change in the characteristics attributable toexternal factors such as temperature and pressure or to internal factorssuch as abrasion of moving parts. Broadly classified, thischaracteristic test is carried out by a calibrator that sets a flowmeterto be tested in a fixed tester to perform the test or by a prover typeflowmeter tester that sets the flowmeter in a fluid system to performthe test.

Since the prover type can perform the flowmeter characteristic test online and perform any characteristic test as needed, it is often used inthe test of an inferential flowmeter especially susceptible to piping,for example, of a turbine meter. The prover has a moving element such asa piston that moves through a pipe with a constant section insynchronism with a fluid and specifies a fluid ejected by the movementof this moving element through a predetermined interval as a referencevolume.

In the flowmeter characteristic test using the prover type, the numberof flow pulses sent per unit volume (flow rate coefficient), a so-calledK factor is calculated from reading of the flowmeter when a fluid of areference volume specified by the prover passes therethrough, that is,by detecting the number of flow pulses sent from the flowmeter.Moreover, if necessary, a continuous flow rate characteristic curve isobtained based on the flow rate coefficients in a plurality of flowrates to be measured.

To obtain the flow rate coefficient at a high resolution, the number offlow pulses sent per reference volume needs to be a predetermined numberor more and, for example, a specified number of 10,000 pulses or more isgiven in the case of a large-sized stationary prover having a largereference volume. On the contrary, if the reference volume is reduced,flow pulses of a specified number or more cannot be sent, but the flowrate coefficient can be obtained from a relationship between thereference volume of fluid ejected as a result of movement of the movingelement such as the piston and the sent pulses (time) sent from theflowmeter during this time. Accordingly, even in the case of the reducednumber of flow pulses, a small-sized prover (small volume prover) isavailable.

In the above small volume prover (hereinafter, referred to as SVP), apiston prover is known that uses a piston as the moving element. Thispiston prover has a measurement cylinder with a constant section that isbasically connected in series with a flowmeter to be tested and comparesthe volume of a fluid displaced when the piston moving in themeasurement cylinder moves through a certain distance with reading ofthe flowmeter at that time. The volume of a fluid is actually obtainedfrom an amount of movement of the piston. In the proving, ordinarily,plural times of test results are averaged to calculate the flow ratecoefficient (K factor) based on the average value. Consequently, thepiston reciprocates by the number of times of the test in themeasurement cylinder for each of the flow rates to be measured.

To return the piston to its original position again after completion ofmeasurement by the movement of the piston through a specified intervalin the measurement cylinder, the piston is driven against flow of afluid by an actuator using a hydraulic pressure or a pneumatic pressurevia a piston rod, in which for a flow passage allowing a fluid to passtherethrough during this time, there are a case where the measurementcylinder itself is used and a case where a bypass flow passageseparately disposed in parallel with the measurement cylinder is used.In the case of causing a fluid to pass through the measurement cylinder,a valve function is provided inside the piston returned by the actuatorso that the valve is closed at the time of the measurement and that thevalve is opened when the piston is returned. This method is called aninternal valve method. Further, in the case of causing a fluid to passthrough the bypass flow passage, a bypass valve is disposed in thebypass flow passage so that the valve is closed at the time of themeasurement and that the valve is opened at the time of the return. Thismethod is called an external valve method.

Since such an SVP ordinarily has the structure having the piston rodintegrally secured to a measuring piston, e.g. when the measuring pistonis subjected to a pressure in a rotating direction, there arises aproblem that the piston rod works as a resistance to impede therotation, imposing a load on the measuring piston. In addition, anexcessive load on this measuring piston prevents the measuring pistonfrom smoothly moving through the measurement cylinder, making accuratemeasurement impossible.

On the other hand, for example, a piston prover described in PatentDocument 1 is proposed. In this piston prover, a measuring piston and apiston rod are separately constructed, and when returning the measuringpiston to a predetermined measurement standby position, the piston rodcauses the measuring piston to move from a downstream side to anupstream side. The measuring piston is set at the predeterminedmeasurement standby position, and thereafter only the piston rod iscaused to move from the upstream side to the downstream side and isaccommodated in a hydraulic cylinder. Thereby, a load on the measuringpiston is tried to be reduced.

PRIOR ART DOCUMENT Patent Document

Patent Document 1: Japanese Patent Publication No. 4782236

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

FIG. 13 is a diagrammatical view for explaining operation of the pistonprover described in Patent Document 1. FIG. 13 (A) depicts a state wherethe measuring piston is at a home position, FIG. 13(B) depicts a statewhere the measuring piston is at a measurement standby position, andFIG. 13(C) depicts a state where the measuring piston startsmeasurement. In the figures, 200 denotes a piston prover, and the pistonprover 200 includes a measurement cylinder 201, a hydraulic cylinder202, a piston rod 203, a measuring piston 204, a flow passage switchingvalve 205, a measurement starting valve 206, a first cylinder inlet 207and a second cylinder inlet 208. As described above, the piston rod 203and the measuring piston 204 are separately constructed.

In FIG. 13 (A), both the flow passage switching valve 205 and themeasurement starting valve 206 are opened with the measuring piston 204being set at the home position. A hydraulic pressure is then applied tothe piston rod 203 in the hydraulic cylinder 202 to move the piston rod203 and move the measuring piston 204 to a predetermined upstreammeasurement standby position for setting the measuring piston 204 at themeasurement standby position. The state at this time is depicted in FIG.13(B). Note that, the piston rod 203 is returned to its originalposition in the hydraulic cylinder 202.

As depicted in FIG. 13(B), the predetermined measurement standbyposition is at an intermediate position between the first cylinder inlet207 and the second cylinder inlet 208. Further, since both the flowpassage switching valve 205 and the measurement starting valve 206 areopened, the measuring piston 204 is kept at the measurement standbyposition by a balance between a pressure of a fluid flown from the firstcylinder inlet 207 and a pressure of a fluid flown from the secondcylinder inlet 208. In addition, as depicted in FIG. 13(C), when boththe flow passage switching valve 205 and the measurement starting valve206 are closed (measurement starting state), the measuring piston 204starts to move from an upstream side to a downstream side to startmeasurement.

However, the above piston prover operates without difficulty when a flowrate is relatively small, but though depending on a size of a cylinderdiameter, for example, in the case of flowing with a large flow ratesuch that a flow velocity is 1 m/s or more, the measuring piston 204moves from the measurement standby position in some cases even thoughnot in the measurement starting state, that is, both the flow passageswitching valve 205 and the measurement starting valve 206 are opened.It is considered that this is caused by that the balance between thepressure of the fluid flown from the first cylinder inlet 207 and thepressure of the fluid flown from the second cylinder inlet 208 isdisrupted to push the measuring piston 204 toward the downstream side inthe case of the large flow rate. More specifically, though themeasurement cylinder 201 is sectioned into two rooms by the measuringpiston 204, the fluid flown from the second cylinder inlet 208 flows inan arrow direction (refer to FIG. 13(B)), and therefore the room on thesecond cylinder inlet 208 side is considered to have a less pressurethan that of the room on the first cylinder inlet 207 side.

The present invention was conceived to correct the above points and itis an object thereof to provide a piston prover which enables accuratemeasurement without imposing an excessive load on the measuring pistonand further, even in the case of the large flow rate, by stopping themeasuring piston at the predetermined measurement standby position.

Means for Solving the Problems

To solve the above problems, a first technical means of the presentinvention is a piston prover comprising a measurement cylinder having anupstream end and a downstream end; a hydraulic cylinder coupled with aside of the downstream end of the measurement cylinder; a measuringpiston that moves through a predetermined distance from an upstream sidetoward a downstream side through the measurement cylinder duringmeasurement due to a fluid flown from the upstream end to eject areference volume of fluid; and a piston rod that is movably accommodatedin the hydraulic cylinder, and having the measuring piston and thepiston rod separately constructed, wherein measurement standby positionstop means that stops the measuring piston at a predetermined upstreammeasurement standby position is included, and when returning themeasuring piston to the predetermined measurement standby position, thepiston rod moves the measuring piston from the downstream side to theupstream side and the measurement standby position stop means stops themeasuring piston at the predetermined measurement standby position,after which only the piston rod is caused to move from the upstream sideto the downstream side and is accommodated in the hydraulic cylinder.

A second technical means is the piston prover of the first technicalmeans, wherein the measurement standby position stop means includes afirst cylinder inflow passage that is connected to an upstream side ofan upstream end side peripheral portion of the measurement cylinder, asecond cylinder inflow passage that is connected to a downstream side ofthe upstream end side peripheral portion of the measurement cylinder, abypass tube by which the first cylinder inflow passage and an upstreamend cover portion of the measurement cylinder are connected, and areturn valve that is provided in the bypass tube, and the predeterminedmeasurement standby position is at a position of the first cylinderinflow passage, and by closing the return valve with the first cylinderinflow passage being blocked by the measuring piston, the measuringpiston is stopped.

A third technical means is the piston prover of the second technicalmeans, wherein the measurement standby position stop means furtherincludes a measurement rod that is provided movably in the upstream endcover portion of the measurement cylinder, and by pushing out themeasuring piston by the measurement rod after opening the return valve,the measuring piston is caused to move from the upstream side to thedownstream side.

A fourth technical means is the piston prover of the first technicalmeans, wherein the measurement standby position stop means includes afirst cylinder inflow passage that is connected to an upstream end coverportion of the measurement cylinder, a second cylinder inflow passagethat is connected to an upstream end side peripheral portion of themeasurement cylinder, a measurement starting valve that is provided inthe first cylinder inflow passage, and a measurement rod that isprovided movably in the upstream end cover portion of the measurementcylinder, the predetermined measurement standby position is at aposition of the first cylinder inflow passage, and by closing themeasurement starting valve with the first cylinder inflow passage beingblocked by the measuring piston, the measuring piston is stopped, and bypushing out the measuring piston by the measurement rod after openingthe measurement starting valve, the measuring piston is caused to movefrom the upstream side to the downstream side.

A fifth technical means is the piston prover of the first technicalmeans, wherein the measurement standby position stop means includes afirst cylinder inflow passage that is connected to an upstream side ofan upstream end side peripheral portion of the measurement cylinder, asecond cylinder inflow passage that is connected to a downstream side ofthe upstream end side peripheral portion of the measurement cylinder, abypass tube by which the first cylinder inflow passage and an upstreamend cover portion of the measurement cylinder are connected, a returnvalve that is provided in the bypass tube, and a measurement startingvalve that is provided in the second cylinder inflow passage, thepredetermined measurement standby position is at a position of the firstcylinder inflow passage, and by closing the return valve and opening themeasurement starting valve with the first cylinder inflow passage beingblocked by the measuring piston, the measuring piston is stopped, and byopening the return valve and closing the measurement starting valve, themeasuring piston is caused to move from the upstream side to thedownstream side.

A sixth technical means is the piston prover of the first technicalmeans, wherein the measurement standby position stop means includes afirst cylinder inflow passage that is connected to an upstream side ofan upstream end side peripheral portion of the measurement cylinder, asecond cylinder inflow passage that is connected to a downstream side ofthe upstream end side peripheral portion of the measurement cylinder, afirst measurement starting valve that is provided in the first cylinderinflow passage, and a second measurement starting valve that is providedin the second cylinder inflow passage, the predetermined measurementstandby position is at an intermediate position between the firstcylinder inflow passage and the second cylinder inflow passage, and byclosing the first measurement starting valve and opening the secondmeasurement starting valve with the measuring piston being at theintermediate position, the measuring piston is stopped, and by openingthe first measurement starting valve and closing the second measurementstarting valve, the measuring piston is caused to move from the upstreamside to the downstream side.

A seventh technical means is the piston prover of any one of the firstto the sixth technical means, wherein the measuring piston has acircumferentially embedded magnetic material, and the measurementcylinder has two detecting means that detect the magnetic materialembedded in the measuring piston at the predetermined distance apartfrom each other on the upstream side and the downstream side of themeasurement cylinder.

An eighth technical means is the piston prover of any one of the firstto the seventh technical means, wherein an openable/closable valvecommunicating with the external air is provided on both the upstream endand the downstream end of the measurement cylinder.

Effect of the Invention

According to the present invention, an excessive load is not imposed onthe measuring piston, and further, even in the case of the large flowrate, the measuring piston can be stopped at the predeterminedmeasurement standby position, thus making it possible to performaccurate measurement.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a front view depicting an external configuration example of apiston prover according to a first embodiment of the present invention.

FIG. 2 is a top view depicting the external configuration example of thepiston prover according to the first embodiment of the presentinvention.

FIG. 3 is a view diagrammatically depicting a configuration example of aproving system including the piston prover according to the firstembodiment of the present invention.

FIG. 4 is a view depicting a configuration example of a measuringpiston.

FIG. 5 is a view diagrammatically depicting a configuration example of apiston prover according to a second embodiment of the present invention.

FIG. 6 is a view diagrammatically depicting a configuration example of apiston prover according to a third embodiment of the present invention.

FIG. 7 is a view diagrammatically depicting a configuration example of apiston prover according to a fourth embodiment of the present invention.

FIG. 8 is a view diagrammatically depicting a configuration example of apiston prover according to a fifth embodiment of the present invention.

FIG. 9 is a view for explaining an initial movement example of thepiston prover according to the present invention.

FIG. 10 is a view for explaining a return movement example of the pistonprover according to the present invention.

FIG. 11 is a view for explaining a standby for measurement operationexample of the piston prover according to the present invention.

FIG. 12 is a view for explaining a measurement action example of thepiston prover according to the present invention.

FIG. 13 is a diagrammatical view for explaining operation of a pistonprover described in Patent Document 1.

PREFERRED EMBODIMENT OF THE INVENTION

Preferred embodiments according to a piston prover of the presentinvention will now be described with reference to the accompanyingdrawings.

First Embodiment

FIGS. 1 and 2 are views depicting an external configuration example of apiston prover according to a first embodiment of the present invention.FIG. 1 is a front view and FIG. 2 is a top view. In the figures, 1denotes a piston prover, 2 denotes a measurement cylinder, 3 denotes anupstream end, 4 denotes a downstream end, 5 and 6 denote air ventvalves, 7 denotes a fluid inlet, 8 denotes a fluid outlet, 9 denotes ameasuring piston, 10 and 11 denote magnetic switches, 12 denotes ahydraulic cylinder, 13 denotes a piston rod, and d1 and d2 denote drains(drain valves).

The piston prover 1 includes the measurement cylinder 2 having theupstream end 3 through which a fluid flows in and the downstream end 4through which the fluid flows out; the hydraulic cylinder 12 coupledwith a side of the downstream end of the measurement cylinder 2; themeasuring piston 9 that moves through a predetermined distance L from anupstream side toward a downstream side through the measurement cylinder2 during measurement due to a fluid flown from the upstream end 3 toeject a reference volume of fluid; and the piston rod 13 that is movablyaccommodated in the hydraulic cylinder 12. The measuring piston 9 ismade of, for example, aluminum and the measurement cylinder 2 is madeof, for example, SUS (stainless steel).

The measuring piston 9 is inserted as a moving element into themeasurement cylinder 2 and, during the measurement, this measuringportion 9 is caused to move by a fluid pressure of a fluid to bemeasured to eject a reference volume of fluid. The piston rod 13 isinserted into the hydraulic cylinder 12 and this piston rod 13 is slidthrough the hydraulic cylinder 12 while being supported in aliquid-tight manner by a journal bearing (not depicted) disposed on anoutflow end face plate making up the downstream end 4. The air ventvalves 5 and 6 and the drains d1 and d2 are openable/closable valvescommunicating with the external air and are properly opened/closedaccording to an action state of the piston prover 1.

The piston prover 1 according to the present invention has the measuringpiston 9 and the piston rod 13 separately constructed, and includesmeasurement standby position stop means 14 that stops the measuringpiston 9 at a predetermined upstream measurement standby position.Moreover, the configuration is such that, when performing a returnmovement of returning the measuring piston 9 to the predeterminedupstream measurement standby position, the piston rod 13 moves themeasuring piston 9 from the downstream side to the upstream side and themeasurement standby position stop means 14 stops the measuring piston 9at the predetermined measurement standby position, after which only thepiston rod 13 is caused to move from the upstream side to the downstreamside and is accommodated in the hydraulic cylinder 12.

The measurement standby position stop means 14 of the present embodimentincludes a first cylinder inflow passage 141 that is connected to anupstream side of an upstream end side peripheral portion 3 b of themeasurement cylinder 2, a second cylinder inflow passage 142 that isconnected to a downstream side of the upstream end side peripheralportion 3 b of the measurement cylinder 2, a bypass tube 143 by whichthe first cylinder inflow passage 141 and an upstream end cover portion3 a of the measurement cylinder 2 are connected, and a return valve 144that is provided in the bypass tube 143. In the case of the presentexample, the predetermined measurement standby position is at a positionof the first cylinder inflow passage 141, and the return valve 144 isclosed with the first cylinder inflow passage 141 being blocked by themeasuring piston 9, thereby stopping the measuring piston 9 at thepredetermined measurement standby position. Note that, the measuringpiston 9 has only to block the first cylinder inflow passage 141, andmay be in contact or not in contact with an inner face of the upstreamend cover portion 3 a of the measurement cylinder 2.

In addition, the measuring piston 9 has a circumferentially embeddedmagnetic material 91. At a position which is spaced apart only apredetermined distance L along a longitudinal direction (upstream sideand downstream side) of the measurement cylinder 2, the measurementcylinder 2 has the magnetic switches 10 and 11 corresponding to twodetecting means that detect the magnetic material 91 embedded in themeasuring piston 9. These two magnetic switches 10 and 11 detect themovement of the measuring piston 9 through the predetermined distance L.That is, the reference volume is a fluid volume ejected by the movementof the measuring piston 9 through the predetermined distance L. Notethat, the predetermined distance L between the magnetic switches 10 and11 is variable so that the reference volume can be adjusted.

FIG. 3 is a view diagrammatically depicting a configuration example of aproving system including the piston prover according to the presentinvention. In the figure, 15 and 16 denote magnetic switches, 17 denotesa pneumatic/hydraulic pressure converting unit, 18 denotes a flowpassage switching valve, 71 denotes a first cylinder inlet, 72 denotes asecond cylinder inlet, 81 denotes a first cylinder outlet, and 82denotes a second cylinder outlet. The fluid inlet 7 is connected to thefirst cylinder inlet 71 via the first cylinder inflow passage 141 andconnected to the second cylinder inlet 72 via the second cylinder inflowpassage 142. Further, the fluid outlet 8 is similarly connected to thefirst cylinder outlet 81 and the second cylinder outlet 82 via acylinder outflow passage. In addition, the fluid inlet 7 and the fluidoutlet 8 are connected to each other via the flow passage switchingvalve 18.

The pneumatic/hydraulic pressure converting unit 17 is configured toinclude a pneumatic pressure supply source 171, a switching valve 172such as a four-way solenoid valve, pneumatic/hydraulic pressureconverting portions 173 and 174, and pressure feed ports 175 and 176. Asupply destination of a pneumatic pressure from the pneumatic pressuresupply source 171 is switched by the switching valve 172 to supply toeither the pneumatic/hydraulic pressure converting portion 173 or thepneumatic/hydraulic pressure converting portion 174. Thepneumatic/hydraulic pressure converting portion 173 converts thepneumatic pressure supplied from the pneumatic pressure supply source171 into a hydraulic pressure and supplies the converted hydraulicpressure through the pressure feed port 175 into the hydraulic cylinder12. This moves the piston rod 13 in a direction being close to themeasurement cylinder 2.

Similarly, the pneumatic/hydraulic pressure converting portion 174converts the pneumatic pressure supplied from the pneumatic pressuresupply source 171 into a hydraulic pressure and supplies the convertedhydraulic pressure through the pressure feed port 176 into the hydrauliccylinder 12. This moves the piston rod 13 in a direction being away fromthe measurement cylinder 2. Namely, the pneumatic/hydraulic pressureconverting unit 17 is a device that introduces a hydraulic pressure intothe hydraulic cylinder 12 or ejects a hydraulic pressure from theinterior of the hydraulic cylinder 12 by opening/closing the pressurefeed ports 175 and 176 so that the piston rod 13 is moved or kept in thehydraulic cylinder 12. Note that, a hydraulic pump may be used insteadof the pneumatic/hydraulic pressure converting unit depending on a portdiameter of the measurement cylinder.

Moreover, a magnetic material not depicted is embedded circumferentiallyaround a head portion making up the piston rod 13 so that it can bedetected by the two magnetic switches 15 and 16 disposed on an outerperiphery of the hydraulic cylinder 12 to locate the position of thepiston rod 13 in the hydraulic cylinder 12.

Specifically, except the return movement of returning the measuringpiston 9 to the predetermined upstream measurement standby position, thepiston rod 13 is in a state of being accommodated in the hydrauliccylinder 12 and, in this state, one end side of the piston rod 13 ispositioned in a vicinity of the magnetic switch 16, thus turning themagnetic switch 16 into an on-state. It is therefore specified if themagnetic switch 16 is in the on-state that the piston rod 13 is in thestate of being accommodated in the hydraulic cylinder 12. Further, inthe above return movement, the piston rod 13 moves in the directionbeing close to the measurement cylinder 2, thus turning the magneticswitch 16 from on to off. It is therefore specified if the magneticswitch 16 is in the off-state that the return movement causes the pistonrod 13 to be in a state of protruding from the hydraulic cylinder 12into the measurement cylinder 2. Then, when the measuring piston 9returns to the measurement standby position by this return movement, themagnetic switch 15 turns from off to on.

As described above, the pneumatic/hydraulic pressure converting unit 17performs a proper hydraulic control according to the state of the pistonrod 13 in the hydraulic cylinder 12 so as to move or keep the piston rod13.

In the piston prover 1 of the present embodiment, as described above,the measuring piston 9 and the piston rod 13 are separately constructedso that during the return movement, the piston rod 13 causes themeasuring piston 9 to move from the downstream side to the upstreamside. At this time, both the flow passage switching valve 18 and thereturn valve 144 are opened. Then, the measurement standby position stopmeans 14 stops the measuring piston 9 at the predetermined upstreammeasurement standby position. Specifically, the return valve 144 isclosed from an open state with the first cylinder inflow passage 141being blocked by the measuring piston 9. This stops the measuring piston9 at the position of the first cylinder inflow passage 141. Note that, afluid flows from the second cylinder inflow passage 142. After stoppingthe measuring piston 9, only the piston rod 13 is then caused to movefrom the upstream side to the downstream side and is accommodated in thehydraulic cylinder 12.

As described above, the measuring piston 9 after the movement is stoppedat the predetermined upstream measurement standby position. In thisstate, the flow passage switching valve 18 is opened and the returnvalve 144 is closed. When starting the measurement, then, by closing theflow passage switching valve 18 and opening the return valve 144, afluid flown from the fluid inlet 7 flows through the bypass tube 143into the measurement cylinder 2. The pressure of this fluid causes themeasuring piston 9 to move to the downstream side to performmeasurement.

Adoption of the above structure enables a smooth movement of themeasuring piston 9 through the measurement cylinder 2 without subjectingthe measuring piston 9 to an excessive load even though the measuringpiston 9 performs rotation, etc. during the measurement, and furtherenables a reliable stopping of the measuring piston 9 at thepredetermined measurement standby position even in the case of a largeflow rate, thus enabling an accurate measurement. Moreover, though alarge and expensive measurement starting valve is provided in the secondcylinder inflow passage in the structure of FIG. 13 described above, thesmall and inexpensive return valve 144 may be provided instead of thismeasurement starting valve in the structure of the present embodiment,thus being advantageous also in terms of cost.

FIG. 4 is a view depicting a configuration example of the measuringpiston. The measuring piston 9 includes the magnetic material 91 such asa magnet, a guide ring 92 made of reinforced Teflon (Teflon: registeredtrademark), etc., an 0 ring 93 made of NBR (nitrile butadiene rubber),etc., a sealing member 94 such as an omni-seal, and a waste tap 95 forpiston fitting. In a conventional detection method of a detector signal,the piston rod secured to the measuring piston was made of invarmaterial with a need to groove this invar material or to apply a ceramiccoating to the entire invar. Hence, it necessitated processing such ascoating surface grinding or attachment of a detector switch, resultingin an increase in the number of manufacturing steps, whereas, thepresent invention has a simplified configuration where the measuringpiston is provided with the magnetic material which is detected by themagnetic switch, thereby enabling a reduction in the number ofmanufacturing steps.

Further, in FIG. 3, at both the upstream end and the downstream end ofthe measurement cylinder 2, the piston prover 1 is provided with the airvent valves 5 and 6 as an example of the openable/closable valvescommunicating with the external air. In addition, either one of the airvent valves 5 and 6 is used to enable a seal check between themeasurement cylinder 2 and the measuring piston 9. In a general sealcheck method, every time the measuring piston performs the returnmovement, a leak from the omni-seal of the measuring piston and theslide (piston) valve was automatically checked as a differentialpressure by applying a pressure to the gap therebetween. In this case, adifferential pressure generator is separately needed, resulting in acomplicated structure which causes troubles such as failures.

A seal check method of the present embodiment performs the leak check bysetting the measuring piston 9, for example, at a predetermined mostdownstream position of the measurement cylinder 2 and manually operatingthe air vent valve 6 while applying a pressure of an actual fluid fromupstream. Further, reversing upstream and downstream, the leak check maybe performed by setting the measuring piston 9 at a predetermined mostupstream position of the measurement cylinder 2 and manually operatingthe air vent valve 5 while applying a pressure of an actual fluid fromdownstream.

In this manner, according to the above seal check method, there is noneed for a part such as the differential pressure generator and it canbe checked using only the air vent valve of the measurement cylinderwhether a leak occurs, thus enabling a reduction in the number of partsand a reduction in the number of manufacturing steps. Moreover, in theparts replacement at the periodic check, etc., the omni-seal of themeasuring piston has only to be replaced, making the maintenance, etc.easy.

Second Embodiment

FIG. 5 is a view diagrammatically depicting a configuration example of apiston prover according to a second embodiment of the present invention.FIG. 5(A) depicts a state where the measuring piston is at the homeposition, FIG. 5(B) depicts a state where the measuring piston is at themeasurement standby position, and FIG. 5(C) depicts a state where themeasuring piston starts measurement. In the first embodiment describedabove, in the case of a large flow rate, when the return valve 144 isopened, the measuring piston 9 starts a movement to start measurement,while in the case of a small flow rate, even when the return valve 144is opened, the measuring piston does not start the movement nor startthe measurement in some cases. Thus, in order to make this better, themeasurement standby position stop means 14 of the present embodimentfurther includes a measurement rod 145 that is provided movably in theupstream end cover portion 3 a of the measurement cylinder 2, inaddition to the first cylinder inflow passage 141, the second cylinderinflow passage 142, the bypass tube 143 and the return valve 144.

This measurement rod 145 is made of, for example, aluminum. At the timeof starting measurement, by pushing out the measuring piston 9 by themeasurement rod 145 after opening the return valve 144, the measuringpiston 9 is caused to move from the upstream side to the downstreamside. This moves the measuring piston 9 smoothly even in the case of asmall flow rate, thus making it possible to start the measurement.

In FIG. 5(A), both the flow passage switching valve 18 and the returnvalve 144 are opened with the measuring piston 9 being set at the homeposition. A hydraulic pressure is then applied to the piston rod 13 inthe hydraulic cylinder 12 to move the piston rod 13 and move themeasuring piston 9 to the predetermined upstream measurement standbyposition, and the measuring piston 9 is stopped at the measurementstandby position. Specifically, the return valve 144 is closed from anopen state with the first cylinder inflow passage 141 being blocked bythe measuring piston 9. This leads to a state where a fluid only fromthe second cylinder inflow passage 142 flows, and the measuring piston 9stops at the position of the first cylinder inflow passage 141. Afterstopping the measuring piston 9, only the piston rod 13 is then causedto move from the upstream side to the downstream side and isaccommodated in the hydraulic cylinder 12. The state at this time isdepicted in FIG. 5(B).

As described above, the measuring piston 9 stops at the predeterminedupstream measurement standby position. In this state, the flow passageswitching valve 18 is opened and the return valve 144 is closed. Then,when starting the measurement, as depicted in FIG. 5(C), by closing theflow passage switching valve 18 and opening the return valve 144, afluid flown from the fluid inlet 7 flows through the bypass tube 143into the measurement cylinder 2. Here, when the measuring piston 9 doesnot start to move, the measuring piston 9 may be pushed out by themeasurement rod 145. This causes the measuring piston 9 to move to thedownstream side to perform measurement.

Third Embodiment

FIG. 6 is a view diagrammatically depicting a configuration example of apiston prover according to a third embodiment of the present invention.In FIG. 6(A), the measurement standby position stop means 14 includes afirst cylinder inflow passage 146 that is connected to the upstream endcover portion 3 a of the measurement cylinder 2, a second cylinderinflow passage 147 that is connected to the upstream end side peripheralportion 3 b of the measurement cylinder 2, a measurement starting valve148 that is provided in the first cylinder inflow passage 146, and ameasurement rod 149 that is provided movably in the upstream end coverportion 3 a of the measurement cylinder 2. Here, the predeterminedmeasurement standby position at which the measuring piston 9 is stoppedis at a position of the first cylinder inflow passage 146, by closingthe measurement starting valve 148 with the first cylinder inflowpassage 146 being blocked by the measuring piston 9, the measuringpiston 9 is stopped, and by pushing out the measuring piston 9 by themeasurement rod 149 after opening the measurement starting valve 148,the measuring piston 9 is caused to move from the upstream side to thedownstream side. Note that, the flow passage switching valve 18 isopened with the measuring piston 9 being stopped and the flow passageswitching valve 18 is closed with the measuring piston 9 being moved.

In the above, the measuring piston 9 stops in an attitude of being incontact with an inner face of the upstream end cover portion 3 a of themeasurement cylinder 2 as depicted in FIG. 6(A). According to the formof FIG. 6(A), the measurement starting valve 148 is provided in thefirst cylinder inflow passage 146 instead of the return valve. Moreover,the first cylinder inflow passage 146 is directly connected to theupstream end cover portion 3 a of the measurement cylinder 2 and hencethe bypass tube is unnecessary.

A modified example of the present embodiment is depicted in FIG. 6(B).In FIG. 6(B), the measurement standby position stop means 14 includes afirst cylinder inflow passage 146′ that is connected to the upstreamside of the upstream end cover side peripheral portion 3 b of themeasurement cylinder 2, the second cylinder inflow passage 147 that isconnected to the downstream side of the upstream end side peripheralportion 3 b of the measurement cylinder 2, the measurement startingvalve 148 that is provided in the first cylinder inflow passage 146′,the measurement rod 149 that is provided movably in the upstream endcover portion 3 a of the measurement cylinder 2, and a bypass tube 150by which the first cylinder inflow passage 146′ and the upstream endcover portion 3 a of the measurement cylinder 2 are connected. Here, byclosing the measurement starting valve 148 with the first cylinderinflow passage 146′ being blocked by the measuring piston 9, themeasuring piston 9 is stopped, and by pushing out the measuring piston 9by the measurement rod 149 after opening the measurement starting valve148, the measuring piston 9 is caused to move from the upstream side tothe downstream side. Note that, the flow passage switching valve 18 isopened with the measuring piston 9 being stopped and the flow passageswitching valve 18 is closed with the measuring piston 9 being moved.

Fourth Embodiment

FIG. 7 is a view diagrammatically depicting a configuration example of apiston prover according to a fourth embodiment of the present invention.The measurement standby position stop means 14 includes a first cylinderinflow passage 151 that is connected to the upstream side of theupstream end side peripheral portion 3 b of the measurement cylinder 2,a second cylinder inflow passage 152 that is connected to the downstreamside of the upstream end side peripheral portion 3 b of the measurementcylinder 2, a bypass tube 154 by which the first cylinder inflow passage151 and the upstream end cover portion 3 a of the measurement cylinder 2are connected, a return valve 155 that is provided in the bypass tube154, and a measurement starting valve 153 that is provided in the secondcylinder inflow passage 152. Here, the predetermined measurement standbyposition at which the measuring piston 9 is stopped is at a position ofthe first cylinder inflow passage 151. By closing the return valve 155and opening the measurement starting valve 153 with the first cylinderinflow passage 151 being blocked by the measuring piston 9, themeasuring piston 9 is stopped. Further, by opening the return valve 155and closing the measurement starting valve 153, the measuring piston 9is caused to move from the upstream side to the downstream side. Notethat, the flow passage switching valve 18 is opened with the measuringpiston 9 being stopped and the flow passage switching valve 18 is closedwith the measuring piston 9 being moved.

The configuration is such that in the present embodiment the measurementrod for pushing out the measuring piston 9 is unnecessary andopening/closing of the measurement starting valve 153 andopening/closing of the return valve 155 are controlled to therebyperform operation of stopping and moving the measuring piston 9.

Fifth Embodiment

FIG. 8 is a view diagrammatically depicting a configuration example of apiston prover according to a fifth embodiment of the present invention.The measurement standby position stop means 14 includes a first cylinderinflow passage 156 that is connected to the upstream side of theupstream end side peripheral portion 3 b of the measurement cylinder 2,a second cylinder inflow passage 157 that is connected to the downstreamside of the upstream end side peripheral portion 3 b of the measurementcylinder 2, a first measurement starting valve 158 that is provided inthe first cylinder inflow passage 156, and a second measurement startingvalve 159 that is provided in the second cylinder inflow passage 157.Here, the predetermined measurement standby position at which themeasuring piston 9 is stopped is at an intermediate position between thefirst cylinder inflow passage 156 and the second cylinder inflow passage157. By closing the first measurement starting valve 158 and opening thesecond measurement starting valve 159 with the measuring piston 9 beingat this intermediate position, the measuring piston 9 is stopped.Further, by opening the first measurement starting valve 158 and closingthe second measurement starting valve 159, the measuring piston 9 iscaused to move from the upstream side to the downstream side. Note that,the flow passage switching valve 18 is opened with the measuring piston9 being stopped and the flow passage switching valve 18 is closed withthe measuring piston 9 being moved.

The configuration is such that in the present embodiment the measurementrod for pushing out the measuring piston 9, the bypass tube, and thereturn valve are unnecessary and opening/closing of the two measurementstarting valves 158 and 159 are controlled to thereby perform operationof stopping and moving the measuring piston 9.

FIGS. 9 to 12 are views for explaining action examples of the pistonprover according to the present invention. FIG. 9 is a view explainingthe initial movement, FIG. 10 is a view explaining the return movement,FIG. 11 is a view explaining a standby for measurement operation, andFIG. 12 is a view explaining the measurement action. Note that, here,though the form of the measurement standby position stop means 14 willbe explained by exemplifying the case of the second embodiment (FIG. 5),basic action is the same even in other embodiments.

In the initial movement of FIG. 9, first, when the measuring piston 9 isat a most upstream position (measurement standby position) P1 instarting a fluid feed from the fluid inlet 7, the piston prover 1 closesthe flow passage switching valve 18 and opens the return valve 144 toallow a fluid to flow in through the second cylinder inflow passage 142and the bypass tube 143. As a result, as depicted in FIG. 9(A), themeasuring piston 9 moves to the downstream side by the fluid pressure.At this time, when the measuring piston 9 does not start to move, themeasuring piston 9 may be pushed out by the measurement rod 145. Notethat, the state of FIG. 9(A) is based on the assumption that theinterior of the measurement cylinder 2 is not filled with fluid.

In addition, when the measuring piston 9 is at a position P2 between themagnetic switches 10 and 11, the flow passage switching valve 18 isclosed and the return valve 144 is opened to allow a fluid to flow inthrough the second cylinder inlet 142, the bypass tube 143, and thefirst cylinder inflow passage 141. As a result, as depicted in FIG. 9(B), the measuring piston 9 moves to the downstream side by the fluidpressure. Similar to the case of FIG. 9(A), the state of FIG. 9(B) isalso based on the assumption that the interior of the measurementcylinder 2 is not filled with fluid.

As depicted in FIG. 9(C), the measuring piston 9 moved as describedabove is set at a downstream home position P3. In both cases of FIG.9(A) and FIG. 9(B), the return valve 144 is already opened and henceremains opened also in FIG. 9 (C). Note that, in all cases, the flowpassage switching valve 18 remains closed. At this home position P3, themeasuring piston 9 is kept at substantially an intermediate positionbetween the first cylinder outlet 81 and the second cylinder outlet 82.

In the return movement of FIG. 10, as depicted in FIG. 10(A), the flowpassage switching valve 18 is opened with the measuring piston 9 beingset at the home position P3. A hydraulic pressure is then applied fromthe pressure feed port 175 to the piston rod 13 to move the piston rod13 as depicted in FIGS. 10(B) and (C) to move the measuring piston 9from the home position P3 to P4 and further to the predeterminedupstream measurement standby position P1, and the measurement standbyposition stop means 14 stops the measuring piston 9 at the measurementstandby position P1. Specifically, the return valve 144 is closed withthe first cylinder inlet 71 being blocked by the measuring piston 9.Here, the movement of the piston rod 13 causes the magnetic switch 16 toturn from on to off. Then, when the magnetic switch 15 turns from off toon, it is determined that the measuring piston 9 has moved to themeasurement standby position P1. At this measurement standby positionP1, the measuring piston 9 is kept at a position of the first cylinderinlet 71.

In the standby for measurement operation of FIG. 11, after themeasurement standby position stop means 14 stops the measuring piston 9at the predetermined upstream measurement standby position P1, ahydraulic pressure is applied from the pressure feed port 176, oppositeto the case of FIG. 10, to the piston rod 13 to return the piston rod 13to its original position in the hydraulic cylinder 12 as depicted inFIGS. 11(A) and (B). The flow passage switching valve 18 is then closedas depicted in FIG. 11(B) to terminate the standby for measurement. Inthis state, the measurement cylinder 2 is filled with fluid. Here, themovement of the piston rod 13 causes the magnetic switch 15 to turn fromon to off. Then, when the magnetic switch 16 turns from off to on, it isdetermined that the piston rod 13 has returned to its original positionin the hydraulic cylinder 12.

In the measurement action of FIG. 12, when the return valve 144 isopened to allow a fluid to flow in through the bypass tube 143, themeasuring piston 9 moves to the downstream side by the fluid pressure asdepicted in FIG. 12(A). Note that, when the measuring piston 9 does notstart to move, the measuring piston 9 may be pushed out by themeasurement rod 145. At this time, when the measuring piston 9 arrivesat a position P5, the magnetic switch 10 is turned on to startmeasurement therefrom. Then, when the measuring piston 9 further movesthrough a position P6 as depicted in FIG. 12 (B) and the measuringpiston 9 arrives at a position P7 as depicted in FIG. 12(C), themagnetic switch 11 is turned on. Consequently, the measuring piston 9 isregarded as having ejected a reference volume of fluid, to terminate themeasurement. The measuring piston 9 is then set at the home position P3depicted in FIG. 9(C) above, after which the return movement of FIG. 10,the standby for measurement operation of FIG. 11, and the measurementaction of FIG. 12 are iteratively executed a required number of times.

As set forth hereinabove, according to the present invention, by virtueof the separate structure of the measuring piston and the piston rod,the measuring piston can move smoothly through the measurement cylinderwithout subjecting the measuring piston to an excessive load inmeasurement, and further the measuring piston can be stopped at thepredetermined measurement standby position reliably, thus enabling anaccurate measurement. Further, by virtue of the simplified structure ascompared with the conventional products, a reduction in the parts countand in cost is achieved.

EXPLANATION OF REFERENCE NUMERALS

1 . . . piston prover, 2 . . . measurement cylinder, 3 . . . upstreamend, 3 a . . . upstream end cover portion, 3 b . . . upstream end sideperipheral portion, 4 . . . downstream end, 5, 6 . . . air vent valve, 7. . . fluid inlet, 71 . . . first cylinder inlet, 72 . . . secondcylinder inlet, 8 . . . fluid outlet, 81 . . . first cylinder outlet, 82. . . second cylinder outlet, 9 . . . measuring piston, 91 . . .magnetic material, 92 . . . guide ring, 93 . . . O ring, 94 . . .omni-seal, 95 . . . waste tap for piston fitting, 10, 11, 15, 16 . . .magnetic switch, 12 . . . hydraulic cylinder, 13 . . . piston rod, 14 .. . measurement standby position stop means, 141,146,146′,151,156 . . .first cylinder inflow passage, 142,147,152,157 . . . second cylinderinflow passage, 143,150,154 . . . bypass tube, 144,155 . . . returnvalve, 145,149 . . . measurement rod, 148,153,158,159 . . . measurementstarting valve, 17 . . . pneumatic/hydraulic pressure converting unit,171 . . . pneumatic pressure supply source, 172 . . . switching valve,173,174 . . . pneumatic/hydraulic pressure converting portion, 175,176 .. . pressure feed port, and 18 . . . flow passage switching valve.

1. A piston prover comprising a measurement cylinder having an upstreamend and a downstream end; a hydraulic cylinder coupled with a side ofthe downstream end of the measurement cylinder; a measuring piston thatmoves through a predetermined distance from an upstream side toward adownstream side through the measurement cylinder during measurement dueto a fluid flown from the upstream end to eject a reference volume offluid; and a piston rod that is movably accommodated in the hydrauliccylinder, and having the measuring piston and the piston rod separatelyconstructed, wherein measurement standby position stop means that stopsthe measuring piston at a predetermined upstream measurement standbyposition is included, and when returning the measuring piston to thepredetermined measurement standby position, the piston rod moves themeasuring piston from the downstream side to the upstream side and themeasurement standby position stop means stops the measuring piston atthe predetermined measurement standby position, after which only thepiston rod is caused to move from the upstream side to the downstreamside and is accommodated in the hydraulic cylinder.
 2. The piston proveras defined in claim 9, wherein the measurement standby position stopmeans includes a first cylinder inflow passage that is connected to anupstream side of an upstream end side peripheral portion of themeasurement cylinder, a second cylinder inflow passage that is connectedto a downstream side of the upstream end side peripheral portion of themeasurement cylinder, a bypass tube by which the first cylinder inflowpassage and an upstream end cover portion of the measurement cylinderare connected, and a return valve that is provided in the bypass tube,and the predetermined measurement standby position is at a position ofthe first cylinder inflow passage, and by closing the return valve withthe first cylinder inflow passage being blocked by the measuring piston,the measuring piston is stopped.
 3. The piston prover as defined inclaim 2, wherein the measurement standby position stop means furtherincludes a measurement rod that is provided movably in the upstream endcover portion of the measurement cylinder, and by pushing out themeasuring piston by the measurement rod after opening the return valve,the measuring piston is caused to move from the upstream side to thedownstream side.
 4. The piston prover as defined in claim 1, wherein themeasurement standby position stop means includes a first cylinder inflowpassage that is connected to an upstream end cover portion of themeasurement cylinder, a second cylinder inflow passage that is connectedto an upstream end side peripheral portion of the measurement cylinder,a measurement starting valve that is provided in the first cylinderinflow passage, and a measurement rod that is provided movably in theupstream end cover portion of the measurement cylinder, thepredetermined measurement standby position is at a position of the firstcylinder inflow passage, and by closing the measurement starting valvewith the first cylinder inflow passage being blocked by the measuringpiston, the measuring piston is stopped, and by pushing out themeasuring piston by the measurement rod after opening the measurementstarting valve, the measuring piston is caused to move from the upstreamside to the downstream side.
 5. The piston prover as defined in claim 1,wherein the measurement standby position stop means includes a firstcylinder inflow passage that is connected to an upstream side of anupstream end side peripheral portion of the measurement cylinder, asecond cylinder inflow passage that is connected to a downstream side ofthe upstream end side peripheral portion of the measurement cylinder, abypass tube by which the first cylinder inflow passage and an upstreamend cover portion of the measurement cylinder are connected, a returnvalve that is provided in the bypass tube, and a measurement startingvalve that is provided in the second cylinder inflow passage, thepredetermined measurement standby position is at a position of the firstcylinder inflow passage, and by closing the return valve and opening themeasurement starting valve with the first cylinder inflow passage beingblocked by the measuring piston, the measuring piston is stopped, and byopening the return valve and closing the measurement starting valve, themeasuring piston is caused to move from the upstream side to thedownstream side.
 6. The piston prover as defined in claim 1, wherein themeasurement standby position stop means includes a first cylinder inflowpassage that is connected to an upstream side of an upstream end sideperipheral portion of the measurement cylinder, a second cylinder inflowpassage that is connected to a downstream side of the upstream end sideperipheral portion of the measurement cylinder, a first measurementstarting valve that is provided in the first cylinder inflow passage,and a second measurement starting valve that is provided in the secondcylinder inflow passage, the predetermined measurement standby positionis at an intermediate position between the first cylinder inflow passageand the second cylinder inflow passage, and by closing the firstmeasurement starting valve and opening the second measurement startingvalve with the measuring piston being at the intermediate position, themeasuring piston is stopped, and by opening the first measurementstarting valve and closing the second measurement starting valve, themeasuring piston is caused to move from the upstream side to thedownstream side.
 7. The piston prover as defined in claim 6, wherein themeasuring piston has a circumferentially embedded magnetic material, andthe measurement cylinder has two detecting means that detect themagnetic material embedded in the measuring piston at the predetermineddistance apart from each other on the upstream side and the downstreamside of the measurement cylinder.
 8. The piston prover as defined inclaim 7, wherein an openable/closable valve communicating with theexternal air is provided on both the upstream end and the downstream endof the measurement cylinder.
 9. A piston prover comprising a measurementcylinder having an upstream end and a downstream end; a hydrauliccylinder coupled with a side of the downstream end of the measurementcylinder; a measuring piston that moves through a predetermined distancefrom an upstream side toward a downstream side through the measurementcylinder during measurement due to a fluid flown from the upstream endto eject a reference volume of fluid; and a piston rod that is movablyaccommodated in the hydraulic cylinder, and having the measuring pistonand the piston rod separately constructed, wherein measurement standbyposition stop portion that stops the measuring piston at a predeterminedupstream measurement standby position by closing a flow passage that ispositioned on an upstream side of the measuring piston is included, whenreturning the measuring piston to the predetermined measurement standbyposition, the piston rod moves the measuring piston from the downstreamside to the upstream side and the measurement standby position stopportion stops the measuring piston at the predetermined measurementstandby position, after which only the piston rod is caused to move fromthe upstream side to the downstream side and is accommodated in thehydraulic cylinder, the measurement standby position stop portionincludes a first cylinder inflow passage that is connected to anupstream side of an upstream end side peripheral portion of themeasurement cylinder, a second cylinder inflow passage that is connectedto a downstream side of the upstream end side peripheral portion of themeasurement cylinder, a bypass tube by which the first cylinder inflowpassage and an upstream end cover portion of the measurement cylinderare connected, and a return valve that is provided in the bypass tube,and the predetermined measurement standby position is at a position ofthe first cylinder inflow passage, and by closing the return valve withthe first cylinder inflow passage being blocked by the measuring piston,the measuring piston is stopped.
 10. The piston prover as defined inclaim 9, wherein the measurement standby position stop portion furtherincludes a measurement rod that is provided movably in the upstream endcover portion of the measurement cylinder, and by pushing out themeasuring piston by the measurement rod after opening the return valve,the measuring piston is caused to move from the upstream side to thedownstream side.
 11. A piston prover comprising a measurement cylinderhaving an upstream end and a downstream end; a hydraulic cylindercoupled with a side of the downstream end of the measurement cylinder; ameasuring piston that moves through a predetermined distance from anupstream side toward a downstream side through the measurement cylinderduring measurement due to a fluid flown from the upstream end to eject areference volume of fluid; and a piston rod that is movably accommodatedin the hydraulic cylinder, and having the measuring piston and thepiston rod separately constructed, wherein measurement standby positionstop portion that stops the measuring piston at a predetermined upstreammeasurement standby position by closing a flow passage that ispositioned on an upstream side of the measuring piston is included, whenreturning the measuring piston to the predetermined measurement standbyposition, the piston rod moves the measuring piston from the downstreamside to the upstream side and the measurement standby position stopportion stops the measuring piston at the predetermined measurementstandby position, after which only the piston rod is caused to move fromthe upstream side to the downstream side and is accommodated in thehydraulic cylinder, the measurement standby position stop portionincludes a first cylinder inflow passage that is connected to anupstream end cover portion of the measurement cylinder, a secondcylinder inflow passage that is connected to an upstream end sideperipheral portion of the measurement cylinder, a measurement startingvalve that is provided in the first cylinder inflow passage, and ameasurement rod that is provided movably in the upstream end coverportion of the measurement cylinder, the predetermined measurementstandby position is at a position of the first cylinder inflow passage,and by closing the measurement starting valve with the first cylinderinflow passage being blocked by the measuring piston, the measuringpiston is stopped, and by pushing out the measuring piston by themeasurement rod after opening the measurement starting valve, themeasuring piston is caused to move from the upstream side to thedownstream side.
 12. A piston prover comprising a measurement cylinderhaving an upstream end and a downstream end; a hydraulic cylindercoupled with a side of the downstream end of the measurement cylinder; ameasuring piston that moves through a predetermined distance from anupstream side toward a downstream side through the measurement cylinderduring measurement due to a fluid flown from the upstream end to eject areference volume of fluid; and a piston rod that is movably accommodatedin the hydraulic cylinder, and having the measuring piston and thepiston rod separately constructed, wherein measurement standby positionstop portion that stops the measuring piston at a predetermined upstreammeasurement standby position by closing a flow passage that ispositioned on an upstream side of the measuring piston is included, whenreturning the measuring piston to the predetermined measurement standbyposition, the piston rod moves the measuring piston from the downstreamside to the upstream side and the measurement standby position stopportion stops the measuring piston at the predetermined measurementstandby position, after which only the piston rod is caused to move fromthe upstream side to the downstream side and is accommodated in thehydraulic cylinder, the measurement standby position stop portionincludes a first cylinder inflow passage that is connected to anupstream side of an upstream end side peripheral portion of themeasurement cylinder, a second cylinder inflow passage that is connectedto a downstream side of the upstream end side peripheral portion of themeasurement cylinder, a bypass tube by which the first cylinder inflowpassage and an upstream end cover portion of the measurement cylinderare connected, a return valve that is provided in the bypass tube, and ameasurement starting valve that is provided in the second cylinderinflow passage, the predetermined measurement standby position is at aposition of the first cylinder inflow passage, and by closing the returnvalve and opening the measurement starting valve with the first cylinderinflow passage being blocked by the measuring piston, the measuringpiston is stopped, and by opening the return valve and closing themeasurement starting valve, the measuring piston is caused to move fromthe upstream side to the downstream side.
 13. A piston prover comprisinga measurement cylinder having an upstream end and a downstream end; ahydraulic cylinder coupled with a side of the downstream end of themeasurement cylinder; a measuring piston that moves through apredetermined distance from an upstream side toward a downstream sidethrough the measurement cylinder during measurement due to a fluid flownfrom the upstream end to eject a reference volume of fluid; and a pistonrod that is movably accommodated in the hydraulic cylinder, and havingthe measuring piston and the piston rod separately constructed, whereinmeasurement standby position stop portion that stops the measuringpiston at a predetermined upstream measurement standby position byclosing a flow passage that is positioned on an upstream side of themeasuring piston is included, when returning the measuring piston to thepredetermined measurement standby position, the piston rod moves themeasuring piston from the downstream side to the upstream side and themeasurement standby position stop portion stops the measuring piston atthe predetermined measurement standby position, after which only thepiston rod is caused to move from the upstream side to the downstreamside and is accommodated in the hydraulic cylinder, the measurementstandby position stop portion includes a first cylinder inflow passagethat is connected to an upstream side of an upstream end side peripheralportion of the measurement cylinder, a second cylinder inflow passagethat is connected to a downstream side of the upstream end sideperipheral portion of the measurement cylinder, a first measurementstarting valve that is provided in the first cylinder inflow passage,and a second measurement starting valve that is provided in the secondcylinder inflow passage, the predetermined measurement standby positionis at an intermediate position between the first cylinder inflow passageand the second cylinder inflow passage, and by closing the firstmeasurement starting valve and opening the second measurement startingvalve with the measuring piston being at the intermediate position, themeasuring piston is stopped, and by opening the first measurementstarting valve and closing the second measurement starting valve, themeasuring piston is caused to move from the upstream side to thedownstream side.
 14. The piston prover as defined in claim 9, whereinthe measuring piston has a circumferentially embedded magnetic material,and the measurement cylinder has two detecting portion that detect themagnetic material embedded in the measuring piston at the predetermineddistance apart from each other on the upstream side and the downstreamside of the measurement cylinder.
 15. The piston prover as defined inclaim 9, wherein an openable/closable valve communicating with theexternal air is provided on both the upstream end and the downstream endof the measurement cylinder.